1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine or printer.
2. Description of the Related Art
A fixing unit in an electrophotographic image forming apparatus such as a copying machine, a printer, and a facsimile apparatus is widely used to fix an unfixed toner image transferred on a sheet such as a transfer sheet and an overhead projector (OHP) sheet onto the sheet. The fixing unit includes a heating member, and a pressure member forming a fixing nip portion together with the heating member. Generally, the fixing unit heats a sheet which bears an unfixed toner image while conveying the sheet by the fixing nip portion, thereby heat-fusing the unfixed toner image to fix it to the sheet.
As the fixing unit, it is ideal that the entire unfixed toner image on the sheet is heat-fused by the heating member to be fixed to the sheet. However, actually, toner remaining unfused or excessively fused toner may be offset from the sheet surface to the heating member. The toner offset from the sheet surface to the surface of the heating member may be transferred from the surface of the heating member to the surface of the pressure member, thereby causing dirt of the surface of the pressure member.
Further, for example, in a heat roller fixing unit or a film heating fixing unit, dirt is more easily accumulated on the surface of the pressure member than on the heating member. The fixing unit heats the surface of the heating member that contacts the unfixed tonner image bearing surface of a sheet to the softening temperature of the toner or higher, to fix the unfixed toner image to the sheet at the time of printing. Therefore, the toner offset to the surface of the heating member to be attached thereto is heated to the softening temperature of the toner or higher at the time of the next printing, and is discharged to the surface of a sheet being conveyed through the fixing unit. Further, paper dusts such as fibers and calcium carbonate of paper, which are attached to the surface of the heating member, are also discharged onto the sheet by adhering to the toner discharged from the surface of the heating member to the surface of the sheet. In this way, the surface of the heating member can easily avoid being cumulatively contaminated.
On the other hand, the surface of the pressure member does not come into contact with the unfixed tonner image bearing surface of the sheet, and therefore the surface of the pressure member does not necessarily have to be heated to the softening temperature of the toner or higher even at the time of printing. Further, it is also desirable in terms of energy saving to refrain from increasing the temperature of the surface of the pressure member more than necessary. Accordingly, in most cases, the temperature of the surface of the pressure member is maintained at a temperature equal to or lower than the softening temperature of the toner at the time of printing. Therefore, the toner attached to the surface of the pressure member is hardly heated to a temperature equal to or higher than the softening temperature so as to have a weak adhesion force to the sheet, whereby the toner on the surface of the pressure member is difficult to be discharged onto a sheet even when the sheet is conveyed through the fixing nip portion at the time of printing.
Further, in a case where paper dusts generated from a sheet is attached to the surface of the pressure member together with the toner, the stickiness is reduced compared to a case where only the toner is attached to the surface of the pressure member, whereby the toner in this case is further difficult to be discharged onto the sheet. For this reason, toner and paper dusts are easily left on the surface of the pressure member, and then are easily accumulated thereon. This phenomenon is especially noticeable at an image forming apparatus lacking an automatic two-sided printing function, according to which a sheet is turned over within the image forming apparatus and then is conveyed to the transfer unit to realize two-sided printing, because, in such an image forming apparatus, the surface of the sheet where the toner is printed rarely comes into contact with the surface of the pressure member.
Japanese Patent Application Laid-Open No. 2000-047509 discusses a method for cleaning a pressure roller by sandwiching a solid white sheet by a fixing nip portion and conveying the sheet while repeating a rotation and a stop. Especially, above method increases the temperature of the surface of the pressure member so that toner contained in dirt on the surface of the pressure member is heated to a temperature equal to or hire than the softening temperature at the time of the stop, thereby increasing the adhesion force, to the sheet, of the dirt to facilitate a discharge of the dirt on the surface of the pressure member onto the sheet.
However, in a case where the dirt attached to the surface of the pressure member contains only a small percentage of the toner, the stickiness as the entire dirt is kept small even if the toner in the dirt is heated to a temperature equal to or higher than the softening temperature to increase the stickiness of the toner in the dirt as described above. Therefore, it is impossible to acquire a sufficient adhesion force, to the surface of the sheet, of the dirt attached to the surface of the pressure member, thereby raising such a problem that the cleaning performance may be insufficient.
Japanese Patent Application Laid-Open No. 3-58074 discusses a method for cleaning a pressure member, in which a sheet with a solid image formed thereon is printed as a cleaning paper for the pressure member to be discharged to the outside of an image forming apparatus, and a user turns over the sheet and provides the sheet through the image forming apparatus again.
Actually, according to the cleaning method discussed in Japanese Patent Application Laid-Open No. 3-58074, even in a case where dirt on the pressure roller contains only a small percentage of toner, it is possible to acquire a large adhesion force of the dirt on the pressure roller to the sheet, and therefore an improved cleaning performance can be expected.
However, when an image forming apparatus does not have the automatic two-sided printing function, according to which a sheet is turned over within the image forming apparatus and then is conveyed to the transfer unit to realize two-sided printing, this method requires a user to participate in the process, thereby raising a problem of deteriorated usability.